Most optical scanning applications use a moving mirror, either rotating or oscillating. A laser beam is typically projected onto the moving mirror so that the beam scans in a specified linear or two-dimensional (2-D) (raster) pattern at a frequency that is sufficient for the particular application. For optical displays, the field of view (FOV) is determined by the scanning amplitude and the particular optical design. There is a minimum frequency (rate) at which scanning displays need to be refreshed that is determined by the human perception of flicker from a scanned display. For ubiquitous raster scanning displays such as cathode ray tubes (CRTs) used in televisions and computer monitors, the display refresh rate is typically 30 Hz or more. Although a CRT is an electron-beam scanning electro-optical display, the same requirements for scan frequency and amplitude (in regard to the FOV) apply for all scanning displays. Thus, for a super video graphics array (sVGA) CRT resolution (i.e., 800 pixels×600 pixels), the minimum horizontal scan rates are 40 kHz for unidirectional scanning and 20 kHz for bidirectional scanning.
Combining both high resolution (i.e., an image with more than 400,000 pixels) and a relatively wide FOV (i.e., greater than 30°) in a single display is a difficult technical challenge, limiting the application of optical scanning for small-size, low cost optical scanners that have both high-resolution and wide FOV. There is a tradeoff between optical scanning frequency versus scanning amplitude (in regard to FOV) for all mirror-scanning devices. The faster the mirror scans, the greater the forces acting on the mirror. Such forces can deform the mirror surface, degrading image quality. This limitation is especially true for small, low-cost resonant mirror scanners. Rotating polygon mirror scanners can overcome this limitation or tradeoff between scan frequency and amplitude, but they are usually bulky, noisy, and costly. In the case of a resonant mirror scanner, the mirror cannot scan more than a few degrees in amplitude at frequencies of 20 kHz to 40 kHz, which is required for sVGA raster scanning displays. Since the optical beam reflects from the scanning mirror, the optical FOV is twice the total mirror deflection angle (i.e., FOV equals 2 times the mirror scan amplitude). However, at sVGA resolution and scan frequencies, an optical FOV on the order of 30 to 60° cannot be achieved using a low-cost resonant mirror scanner to produce micro-displays.
Accordingly, it will be apparent that a different approach to optical scanning is needed to produce both an acceptable resolution and FOV within a compact assembly. The device should also be low in cost and relatively simply to produce.